Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene, and carbon nano-tubes (and its larger-diameter cousins—carbon nano-fibers or CNFs). The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. A graphene sheet or basal plane is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a hundred nanometers. Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, and as composite reinforcements.
However, CNTs are extremely expensive due to the low yield and low production rates commonly associated with all of the current CNT preparation processes, such as arc discharge, laser ablation, chemical vapor deposition (CVD), and catalytic CVD (CCVD). The high material costs have significantly hindered the widespread application of CNTs. Earlier CNT production methods include those disclosed in the following patents: H. G. Tennent, “Carbon Fibrils, Method for Producing Same and Compositions Containing Same,” U.S. Pat. No. 4,663,230 (May 5, 1987); C. Snyder, “Carbon Fibrils,” U.S. Pat. No. 5,707,916 (Jan. 13, 1998).
Carbon nano-fibers (CNFs) are prepared from CVD, CCVD, or electro-spinning of polymer nano-fibers followed by carbonization. Electro-spinning has not been regarded as a mass-production method due to the limited amount of material that can be electro-spun with one hollow needle head. An example of the process to produce polymer nano-fibers via electro-spinning is given in D. H. Reneker, et al, “Processes for Producing Fibers and Their Use,” US Pub. No. 2009/0039565 (Feb. 12, 2009).
The CNFs produced by the CVD and CCVD processes are commonly referred to as vapor-grown carbon nano-fibers (VG-CNFs). VG-CNFs have been extensively investigated in recent years and are commercially available at very high prices (e.g., $300/Kg). The following are some examples of CNF production processes: D. J. C. Yates, et al., “Production of Carbon Filaments,” U.S. Pat. No. 4,565,683 (Jan. 21, 1986); S. H. Yoon, “Ultra-fine Fibrous Carbon and Preparation Method Thereof,” US Pub. No. 2009/0075077 (Mar. 19, 2009); S. H. Yoon, “Ultra-fine Fibrous Carbon and Preparation Method Thereof,” U.S. Pat. No. 7,470,418 (Dec. 30, 2008); S. H. Yoon, “Porous Filamentous Nano Carbon and Method of Forming the Same,” US Pub. No. 2009/0004095 (Jan. 1, 2009); G. Oriji, “Carbon Nano Fiber, Production and Use,” US Pub. No. 2009/0008611 (Jan. 8, 2009); J. L. Gonzales Moral, et al., “Carbon Nanofibers and Procedure for Obtaining Said Nanofibers,” US Pub. No. 2009/0035569 (Feb. 5, 2009).
VG-CNFs and related CNTs have several drawbacks that have significantly constrained their scope of application:                (a) Both CVD and CCVD processes typically involve using a catalyst and the catalyst particles (e.g., transition metal nano particles or their alloys) usually become part of the resulting CNF or CNT structure. Normally, there is a significant amount of catalyst used in these processes. The residual catalyst, even just a trace amount, is considered undesirable in many applications. For instance, Fe is viewed as detrimental to the performance of a lithium ion battery if CNFs or CNTs are used as an anode active material. Catalytic particles can also catalyze or accelerate thermal or chemical degradation of a polymer matrix composite material.        (b) The CVD or CCVD processes intrinsically introduce a significant amount of impurities into the resulting CNFs or CNTs. It is not unusual to find a purity level (graphitic carbon content) in a CNF less than 80-90%.        (c) Depending upon the processing conditions, the graphene planes in different CNFs may be oriented at different angles with respect to the fiber axis. Furthermore, the graphene planes may be curved as a cup-shape or a cone-helix structure, which are not conducive to achieving high strength or modulus along the fiber axis. In one example, the fibers consist primarily of conical nano-fibers, but can contain a significant amount of bamboo nano-fibers. Most conical nano-fibers consist of an ordered inner layer and a disordered outer layer. When subjected to a thermal treatment above 1,500° C., some CNFs can undergo a structural transformation with the ordered inner layers changing from a cone-helix structure to a highly ordered multiwall stacked cone structure. The bamboo nano-fibers can have a tapered multiwall nanotube structure for the wall and a multi-shell fullerene structure for the cap of each segment, surrounded by a disordered outer layer. When these fibers are heat treated, the disordered outer layers transform to an ordered multiwall nanotube structure and merge with the wall of each segment. The end caps of each segment transform from a smooth multiwall fullerene structure to one consisting of disjointed graphene planes. Such a thermally induced instability in the CNF structure is an undesirable feature of CNFs for high-temperature applications (e.g., as a reinforcement in a carbon matrix composite).        (d) The CNFs typically have a continuous thermal carbon overcoat, which is a result of the thermal decomposition effect during the CNF formation process via the CVD, CCVD, or carbonization of electro-spun polymer nano-fibrils. Although this carbon overcoat could serve as a protective layer for the internal graphitic crystallites in some applications, the overcoat is detrimental to many other engineering applications. For instance, this overcoat makes it difficult to chemically functionalize the CNF surface, thereby inhibiting the formation of a strong bond between a CNF and a polymer matrix in a polymer composite. This hard overcoat also makes it difficult for lithium ions to enter or leave the CNF if the CNF is used as a lithium ion battery anode material. In a similar manner, a CNT has a complete, continuous graphene plane wrapped around the tube axis, which has few active sites where chemical functionalization can occur. Hence, chemical functionalization occurs only at the edge unless this surface is chemically treated (e.g., with a strong oxidizing agent, such as fuming sulfuric acid and nitric acid).        (e) In most of the VG-CNFs, the graphene planes or graphitic crystallites are oriented at a non-zero angle with respect to the fiber axis, resulting in a lower strength, modulus, thermal conductivity, and electrical conductivity along the fiber axis direction as compared with fibrils having all graphene planes substantially parallel to the fiber axis (e.g., CNTs).        
Hence, it is desirable to have a sub-micron-scaled carbon or graphite fiber or a lower-micron graphitic fibril (diameter<6 μm, but ≧1 μm) that has a well-controlled, consistent, and stable structure to ensure consistent properties and performance. It is further desirable to have a low-cost process that is capable of producing graphitic fibrils in large quantities. It is also desirable to have graphitic fibrils that are pure and catalyst-free. For composite material applications, it is still further desirable to have graphitic fibrils that exhibit much more surface areas for chemical functionalization or interactions with a chemical species or a matrix material. It is most desirable to have a graphitic fibril that exhibits a higher strength, modulus, thermal conductivity, and electrical conductivity as compared with conventional CNFs.
For lithium ion battery anode applications, it is desirable to have graphitic fibrils that have proper diameters, sufficiently small to ensure easy migration of lithium ions in and out of the fibrils to enable high-rate capability, yet sufficiently small to ensure a minimal amount of solid-electrolyte interface (SEI) that would irreversibly consume lithium initially stored in the cathode. The main object of the present invention is to provide submicron graphitic fibrils that exhibit these desirable attributes. It is another object of the present invention to provide lower-micron graphitic fibrils (≧1 μm, but <6 μm) that have desirable characteristics to serve as an anode active material for the lithium ion battery.
Concerns over the safety of earlier lithium metal secondary batteries led to the development of lithium ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials (soft carbon, hard carbon, graphite, carbon/graphite fibers, etc.) as the anode. A graphite or carbon material can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1. In order to minimize the loss in energy density, x in LixC6 must be maximized and the irreversible capacity loss Qir in the first charge/discharge cycle of the battery must be minimized.
Carbon or graphite anodes can have a long cycle life due to the presence of a protective surface-electrolyte interface layer (SEI), which results from the reaction between lithium and the electrolyte during the first several cycles of charge-discharge. The lithium in this reaction comes from some of the lithium ions originally stored in the cathode intended for the charge transfer purpose. As the SEI is formed, the lithium ions become part of the inert SEI layer and become irreversible, i.e, they can no longer be the active element for charge transfer. Therefore, it is desirable to use a minimum amount of lithium for the formation of an effective SEI layer. In addition to SEI formation, Qir of natural graphite has been attributed to graphite exfoliation caused by electrolyte solvent co-intercalation and other side reactions. In the prior art, in order to prevent such an electrolyte-induced exfoliation, particles of natural graphite are typically coated with a layer of amorphous carbon. However, such a coating effectively reduces the proportion of graphite for reversibly storing lithium. It may be noted that the specific capacity of an anode is calculated according to the lithium storage capacity divided by the total anode weight, which is the sum of the anode active material, active material surface coating, conductive filler, and binder weights. If the amount of non-active materials (surface coating, filler, and binder) can be reduced or eliminated, the proportion of anode active material in the anode can be significantly increased.
The maximum amount of lithium that can be reversibly intercalated into the interstices between graphene planes of a perfect graphite crystal is generally believed to occur in a graphite intercalation compound represented by LixC6 (x=1), corresponding to a theoretical specific capacity of 372 mAh/g. In other graphitized carbon materials than pure graphite crystals, there exists a certain amount of graphite crystallites dispersed in or bonded by an amorphous or disordered carbon matrix phase. This amount of disordered carbon material reduces the effective lithium storage capacity to typically <360 mAh/g, more typically <350 mAh/g, and, in many cases, <330 mAh/g (e.g., meso-carbon micro-beads, MCMBs, a very commonly used anode active material for electric vehicle power applications).
Hence, it is desirable to have a new graphitic material that has a maximum amount of perfect graphene crystal structure (with a minimum amount of disordered structure), requiring no external coating and conductive additive, having an optimal diameter (small enough to enable easy lithium entry and extraction, but large enough to have a minimal specific surface area and, hence minimal amount of SEI), resulting in exceptional specific capacity and cycling stability. Most surprisingly, the presently invented submicron graphitic fibrils (0.1 μm<diameter<1 μm) and lower-micron graphitic fibrils (1 μm≦diameter<6 μm), prepared by intercalating, exfoliating, and separating graphite or carbon fibers, exhibit all of the above desirable characteristics. This has not been taught or implied in the prior art.